1. Field of the Invention
This application relates generally to optical diagnostic methods and apparatus and, more specifically, to a method and apparatus that identifies a feature within a region of the retina, and optionally estimates a volume of a portion of that region.
2. Description of Related Art
In ophthalmic and other applications, optical coherence tomography (“OCT”) is frequently used to generate three-dimensional scan data of a volume at a location being examined. An example of such a location is the optic nerve head (“ONH”), also referred to as the optic disc, which is where ganglion cell axons, also commonly referred to as nerve fibers, come together and exit the eye to form the optic nerve. The ONH is the distal portion of the optic nerve extending from the myelinated portion of nerve that begins just behind the sclera, to the retinal surface. The optic disc is also the entry point for the major blood vessels that supply blood to the retina, so a large amount of information useful in medical diagnoses can be extracted from this location.
The structural uniqueness of such locations can offer a useful indication of the eye's health and an early indication of the onset or progression of certain conditions. For example, optic nerve damage or atrophy can be indicative of glaucoma or another serious disease of the retina causing a large amount of neural tissue to be destroyed. Similarly, swelling of the optic nerve caused by inflammatory changes in the nerve, a condition known as optic neuritis, may be a manifestation of multiple sclerosis. Detection of each of these symptoms can be facilitated through the identification of the ONH and segmentation of the layers of the retina. However, the significant structural difference of the ONH as compared to neighboring retinal areas makes layer segmentation of ONH images difficult. And the structural variations of the ONH among subjects add to the difficulty of accurately using layer segmentation to detect regions of interest, such as the ONH cutout region.
Layer segmentation performed on ONH images obtained through OCT has traditionally ignored the ONH region (segmented through the ONH region using the same algorithm and parameters). A following step is needed, if possible, to identify the location of an ONH cutout region utilizing the layer segmentation information from a full three-dimensional dataset. The segmentation results in the detected ONH cutout region are then erased and excluded from the following analysis. However, acute structural changes approaching the ONH often result in segmentation errors or deviations in one or more layer boundaries adjacent to the ONH, which are still present even with a post-processing procedure that erases segmented boundaries within the ONH after identifying the ONH cutout from the three-dimensional data.
Another approach to layer segmentation performed on ONH images has involved masking a location where the cutout region is believed to be located and then analyzing the unmasked regions. According to this approach, a mask having a suitably-large size and shape to ensure that even the largest cutout regions will be masked so the remainder of the ONH appearing in the images can be analyzed for layer segmentation purposes. However, such an arbitrary-sized mask must be large enough to conceal the cutout region of various different subjects. As a result the mask will often be too large for the subject, excluding regions of the ONH of interest from the analysis. These excluded regions can contain information that would be useful in diagnosing a condition related to the eye.